1. Field of Invention
The present invention is a new process for growing spin-capable multi-walled nanotube (MWCNT) forests in a repeatable fashion.
2. Background of the Invention
Since the discovery that one can directly pull multi-walled carbon nanotubes (MWCNTs) into the forms of sheets and/or yarns from forests [Jiang K, Li Q, Fan S. Spinning continuous carbon nanotube yarns. Nature 2002; 419:801 (“Jiang 2002”)], the majority of research effort has been focused on the physical, mechanical and electrical properties of these MWCNT yarns and sheets as well as their various applications [Zhang M, Atkinson K R, Baughman R H. Multifunctional carbon nanotube yarns by downsizing an ancient technology. Science 2004 306:1358-1361 (“M Zhang 2004”); Zhang X, Jiang K, Feng C, Liu P, Zhang L, Kong J, et al. Spinning and processing continuous yarns from 4-Inch wafer scale super-aligned carbon nanotube arrays. Adv Mater 2006 18:1505-1510 (“X Zhang 2006”); Zhang X, Li Q, Tu L, Li Y, Coulter J Y, Zheng L, et al. Strong carbon-nanotube fibers spun from long carbon-nanotube arrays. Small 2007 3:244-248 (“X Zhang 2007”); Li Q, Li Y, Zhang X, Chikkannanavar S B, Zhao Y, Dangelewicz A M, et al. Structure-dependent electrical properties of carbon nanotube fibers. Adv Mater 2007 19:3358-3363 (“Li 2007”); Liu K, Sun Y, Chen L, Feng C, Feng X, Jiang K, et al. Controlled growth of super-aligned carbon nanotube arrays for spinning continuous unidirectional sheets with tunable physical properties. Nano Lett 2008 8:700-705 (“Liu 2008”)]. In particular, many applications have been demonstrated that are only possible because of the unique properties of CNTs [Baughman R H, Zakhidov A A, de Heer W A. Carbon nanotubes—the route toward applications. Science 2002 297:787-792 (“Baughman 2002”)]. These potential applications are widespread and include transparent electrodes, flexible displays and composite materials [Baughman 2002; Zhang M, Fang S, Zakhidov A A, Lee S B, Aliev A E, Willaims C D, et al. Strong, transparent, multifunctional, carbon nanotube sheets. Science 2005 309:1215-1219 (“M Zhang 2005”); Ulbricht R. Polymeric solar cells with oriented and strong transparent carbon nanotube anode. Phys Stat Sol B 2006 243:3528-3532; Gruner G. Carbon nanotube films for transparent and plastic electronics. J Mater Chem 2006 16:3533-3539; Kaempgen M, Duesberg G S, Roth S. Transparent carbon nanotube coatings. Appl Surf Sci 2005 252:425-429; Cheng Q, Wang J, Jiang K, Li Q, Fan S. Fabrication and properties of aligned multi-walled carbon nanotube-reinforced epoxy composites. J Mater Res 2008 23:2975-2983].
Applicant believes much less research effort has centered on the critical growth factors that determine whether or not the MWCNTs in a forest can be pulled or spun. These factors remain somewhat obscure to date because only a few research groups [Jiang 2002; M Zhang 2004; X Zhang 2006; X Zhang 2007] have actively investigated the protocols to grow “spin-capable” MWCNTs over the past half decade. While those groups have used different growth conditions (catalysts, gases and temperatures) the data and understanding still need improvement. For example: while it is well known that MWCNTs will grow on Fe or Al2O3/Fe using ethylene or acetylene as a carbon source and argon, helium, and/or hydrogen as the carrier gas, the most important factors for growing spin-capable forests are not well known. These still need to be investigated further. Some reports [Jiang 2002; X Zhang 2006; Li 2007; Liu 2008, Li Q, Zhang X, DePaula R F, Zheng L, Zhao Y, Stan L, et al. Sustained growth of ultralong carbon nanotube arrays for fiber spinning Adv Mater 2006 18:3160-3163] suggest that the spinning capabilities of MWCNT forests result when the CNTs in the forest become “super-aligned” arrays. The idea is that the CNTs must be very well aligned parallel to one another and in addition have a high density in the forest.
Further study also indicates that super-aligned CNTs are generally formed when there is a high CNT nucleation density (or high density of catalyst nanoparticle sites) [Liu 2008; Nessim G D, Hart A J, Kim J S, Acquaviva D, Oh J, Morgan C D, et al. Tuning of vertically-aligned carbon nanotube diameter and areal density through catalyst pre-treatment. Nano Lett 2008 8:3587-3593, 15; Futaba D N, Hata K, Namai T, Yamada T, Mizuno K, Hayamizu Y, et al. 84% catalyst activity of water-assisted growth of single walled carbon nanotube forest characterization by a statistical and macroscopic approach. J Phys Chem B 2006 110:8035-8038]. As a consequence, there have been some significant efforts to increase the nucleation density by controlling both the size and size distribution of Fe catalyst nanoparticles through various methods. These methods have included gas-assisted pretreatment methods [Cantoro M, Hofmann S, Pisana S, Ducati C, Parvez A, Ferrari A C, et al. Effects of pre-treatment and plasma enhancement on chemical vapor deposition of carbon nanotubes from ultra-thin catalyst films. Diamond Relat Mater 2006 15:1029-1035; Zhang G, Mann D, Zhang L, Javey A, Li Y, Yenilmez E, et al. Ultra-high-yield growth of vertical single-walled carbon nanotubes: hidden roles of hydrogen and oxygen. Proc Natl Acad Sci USA 2005 102:16141-16145; Pisana S, Cantoro M, Parvez A, Hofmann S, Ferrari A C, Robertson J. The role of precursor gases on the surface restructuring of catalyst films during carbon nanotube growth. Physica E 2007 37:1-5] the use of various catalysts [Wang Y, Luo Z, Li B, Ho P S, Yao Z, Shi L, et al. Comparison study of catalyst nanoparticle formation and carbon nanotube growth: Support effect. J Appl Phys 2007 101:124310 (“Wang 2007”); Hofmann S, Cantoro M, Kleinsorge B, Casiraghi C, Parvez A, Robertson J, et al. Effects of catalyst film thickness on plasma-enhanced carbon nanotube growth. J Appl Phys 2005 98:034308], and the optimization of the catalyst film thickness [Chakrabarti S, Kume H, Pan L, Nagasaka T, Nakayama Y. Number of walls controlled synthesis of millimeter-long vertically aligned brushlike carbon nanotubes. J Phys Chem C 2007 111:1929-1934; Patole S P, Alegaonkar P S, Shin H C, Yoo J B. Alignment and wall control of ultra long carbon nanotubes in water assisted chemical vapour deposition. J Phys D: Appl Phys 2008 41:155311].
It is therefore important to study how the nucleation density is affected by controlling the size and size distribution of Fe catalyst particles and optimizing gas flow rates used in the growth of CNTs. This information is valuable in determining optimal conditions for developing super-aligned arrays of CNTs as well as improving the ability to pull CNTs out from CNT forests. Understanding the mechanism by which super-aligned arrays and high nucleation density can be achieved is critical for controlling and stabilizing the growth process for the spinning CNTs as well as providing a bigger opportunity ranging from academic research to commercial applications of the CNT sheets and yarns.